The Difference Between Nil, True, False, Blank and Empty in Ruby and Ruby on Rails
I would like to start my first post with a topic that i think is quite sensitive in Ruby. This is about recognizing the differences and similarities of a Nil, True, False, Blank and Empty expression. When i experiment with these values, i always like to open up a rails console, so before doing anything, open up a console by typing (rails c is short for rails console) :
Checking The Expression With ?
Ruby uses the english question mark to check the validity of an expression. For example :
irb(main):002:0> 'a'.nil? => false
checks whether ‘a’ is nil, which of course is false.
The Nil Expression
It’s pretty important that you understand the difference of these expressions. To begin, let’s start with nil. Nil is the ruby way of saying NULL. Whenever there is an expression that returns nothing, we get a nil that specifies that. For instance, in a find_by, if the record is not there we get nil as a result. Notice that find returns RecordNotFound exception. As everything is an object in Ruby, Nil is also an object (of class NilClass). Nil is not the same as false.
In Ruby (and Rails of course), every expression that is not nil or false is considered true. False and True expressions work pretty much as expected, but pay attention to the fact that, again, they are objects (TrueClass, FalseClass). Take a look at the console below :
irb(main):053:0> 0==1 => false irb(main):054:0> 1==1 => true irb(main):055:0> nil==true => false irb(main):056:0> nil==false => false irb(main):057:0> 0==true => false irb(main):058:0> 0==false => false irb(main):060:0> true.class => TrueClass
Nil is not either true or false. 0 is not either true or false(or nil). It’s just 0.
The Blank? Expression
Blank is a Rails expression that checks whether an object is either nil or false. Notice that blank evaluates correctly if the object is a space padded string.
irb(main):066:0> ''.blank? => true irb(main):067:0> nil.blank? => true irb(main):068:0> ' '.blank? => true irb(main):069:0> 'a'.blank? => false irb(main):070:0> false.blank? => true irb(main):071:0> true.blank? => false irb(main):073:0> .blank? => true irb(main):074:0> ['a'].blank? => false
The Empty? Expression
Empty can only be used in some objects like an array or a string. As you can guess, it checks whether an object is empty (empty string, no elements array etc).
irb(main):078:0> ' '.empty? => false irb(main):079:0> ''.empty? => true irb(main):080:0> .empty? => true irb(main):081:0> ['a'].empty? => false irb(main):082:0> nil.empty? NoMethodError: You have a nil object when you didn't expect it! You might have expected an instance of Array. The error occurred while evaluating nil.empty? from (irb):82 from :0
A space padded string is not empty, although it’s blank. Also notice that nil.empty? is an incorrect expression, because the nil object does not have an empty method.